The present invention relates to synchronizing signal separator circuits, and more particularly to a transistor circuit for preventing a synchronizing signal separating circuit from deriving an output in response to a noise pulse contained in a composite video signal.
A synchronizing signal separating circuit in a television set or the like separates a synchronizing signal from a composite video signal which contains a video information signal and the synchronizing signal. Though various types of synchronizing signal separating circuits have been heretofore proposed, the synchronizing signal separating circuit disclosed in U.S. Pat. No. 3,879,576 issued on Apr. 22, 1975 can achieve a preferable separating operation irrespective of the DC level change of the composite video signal due to variation of the electric field strength or the amplitude of the video information signal. This known circuit comprises a transistor for signal separation, a capacitor and a constant-current source. The composite video signal is applied via the capacitor to the emitter of the separating transistor. The emitter of the transistor is further connected to the constant-current source. A bias voltage is applied to the base of the transistor and an output is derived from its collector. Accordingly, during the synchronizing signal period in the composite video signal, the transistor is made conductive and charges the capacitor so that the synchronizing signalis outputted from the transistor. During the subsequent video information signal period, the transistor becomes non-conductive, and the electric charges in the capacitor are discharged by the constant-current source. In this process, the total charged quantity and the total discharged quantity of electric charges of the capacitor in one cycle of the composite video signal are made equal to each other. In addition, the charging current to the capacitor is chosen sufficiently larger than the discharging current. By making such provision, the transistor is made conductive only during the synchronizing signal period to fix the peak potential level of the synchronizing signal substantially to the voltage level obtained by subtracting the base-emitter voltage of the transistor from its bias voltage. At this time, the emitter bias voltage of the transistor is not charged by the input composite video signal. As a result, the transistor is made conductive only during the synchronizing signal period. Thus, there is provided a synchronizing signal separating circuit which can achieve a correct synchronizing signal separating operation without being influenced by the DC level change of the composite video signal due to the variation of the electric field strength or the amplitude of the video information signal.
However, if a noise pulse having larger amplitude than that of the synchronizing signal is contained in the composite video signal, the transistor for separating the synchronizing signal would respond to this noise pulse to generate the noise output other than the synchronizing signal output. This noise output would cause the disturbance in the vertical synchronization and malfunction of the AFC (automatic frequency control).